Method and composition for forming precision molds



United States Patent 3,164,872 METHOD AND COMPQSITHON FGR FGRMENG PRECISIGN MOLDS Herbert Greenewaid, Jr., Dallas, Tex. (4296 Braunton Road, Columbus, Ohio) No Drawing. Filed Nov. 30, 1962, Ser. No. 241,121 11 flaims. .(Cl. 22-193) This invention relates to a method of and composition for forming articles especially adapted for use as precision casting molds.

Ceramic molds for foundry use heretofore have bven formed either from a cast slurry consisting of a refractory powder and a quick-setting ethyl silicate bond, or press formed from a damp powder. The cast slurry process is relatively expensive and has a number of serious disadvantages. The process is limited to the case where only small batches of material can be mixed at a time, i.e., is characterized by having a short pot life. Further, the slurry has a tendency to shrink during curing and this sets up stresses within the mold which contribute to warpage of the mold during the firing operation. variety of compositions have been proposed for the damp powder employed in the press forming process but all of such compositions known to date have embodied serious limitations. The best of the pressed compositions are deficient in that they have a relatively low over-all strength and tend to have a surface having a hardness which is disproportionate with respect to the hardness of the material at the center. The reason for this is that the water soluble bonding agents added to the molding material have an excessive tendency to migrate to the mold surface during the curing and drying operations. This makes it difiicult to maintain adequate mold strength in the center of the mold without reducing the fusion point of the mold surface below the minimum permissible for the precision casting of steels or other high melting point alloys. This same lack of strength in the center of molds made by prior practice makes it impossible to obtain a suitably "smooth surface on any machined surfaces of the mold. As a result, it is impossible to make precision castings in molds formed by machining cavities into cured blocks of compacted molding material produced by prior art.

Accordingly, the main object of the present invention is to provide a material particularly adapted to foundry use which overcomes the above disadvantages.

A further object is to provide an agglomerate for forming articles of the above type in which warpage and shrinkage are largely eliminated.

Another object is to provide a mold having improved strength permitting the economic production and use of larger size molds. e

Another object is to provide a mold having a hard, wear-resistant surface and yet having a hard center, thus permitting the machining of intricate molds from blocks of pressed and cured material.

Another object is to provide a bonding system capable of'use with refractory powdered materials having higher melting points than those obtainable with oxide refractories.

Another object is to provide a bonding system capable of use with oxide refractory powders, with metal powders, with high melting point carbide, boride, nitride, and with other powders with equal facility. 7

Still another object of this invention isto provide a mold suit-able for use in the precision casting of metals having greater dimensional stability and greater and more uniform strength than has been possible heretofore.

According to one embodiment of the present invention, a damp agglomerate material is formed consisting of the following ingredients in the preferred proportions as stated, all parts being by weight:

1000 parts of a refractory powder such as silica flour (200 mesh preferred) 40 parts nonacidic bentonite 4 parts sodium hydroxide 20 parts silicon metal powder prefired parts water The use of a refractory powder of silica flour of 200 mesh is indicated above as being preferred. Other refractory powders, however, may be employed. Where they are employed, the amount in parts by weight of such refractory powder will be provided in proportion to the ratio of the specific gravity of that refractory powder to the specific gravity of the silica fiour. This proportionate weight of refractory powders other than silica flour will be hereinafter referred to as the silica flour equivalent weight.

In actual practice'in this preferred embodiment, the refractory powder, a prefired silicon metal selected from the class of prefired silicon metal powders, and the bentonite are thoroughly mixed in the dry state to form a dry composite. The sodium hydroxide is'then thoroughly dissolved in the water and the resulting solution is added to the dry composite and thoroughly mixed. The mixed material is firmly pressed or hard rammed into molds about a pattern, preferably metal, and within a rigid flask, such as one made of steel. The rammed or pressed molds are then removed from the pattern plate and set on a flat, ground, rigid plate, the flask is removed, and the mold is cured at a temperature of about 400 F. for a number of hours proportionate to the mold size. The molds will core at room temperature, but will require a much longer time to cure. The time for curing atroom temperature is dependent on the temperature of prefiring of the silicon metal powder. For example, if the silicon metal powder has been prefired at 1800 F. for 16 hours, it will take about 8 hours for the curing reaction 'to start at room temperature and this curing reaction will be complete in about 24 hours. On the other hand, if the silicon metal .powder is used in the unfired condition, it is difiicult, if not impossible, to mix the material thoroughly and place it in the mold flask before curing takes place.

After the molds are cured, they are assembled into stacks, preheated, and poured. For the casting of alumium, magnesium, copper base alloys, and'cast iron, preheating is normally omitted. However, in such case virtually all free moisture must be dried from the mold before molten metal is introduced, because the permeability of this mold is insuflicient to permit the complete escape through the mold walls of steam formed by the heat of the molten metal as it is poured into the mold. For the casting of steel, nickel, cobalt, and other high melting point alloys, the molds must be preheated if good surface finish is to be obtained on the castings. Mold preheat temperatures Will'normally be within the range of from room temperature to 2400 F.

The bonding system of the present invention involves a mixture of nonacidic bentonite, commonly known as Western'bentonite, with sodium'hydroxide and fired silicon metal powder in closely controlled amounts. A chemical reaction between these materials results in a solid mold of uniform density and desirable qualities as to strength. The chemical reaction may be accelerated by heat to form a chemical type ceramic bond in situ, probably consisting of a high silica content sodium alumino-silicate. This system can be equally applied to the bonding of refractory. powders of a wide variety of compositions. Examples of such refractory powders are: silica, alumina, zircon, zirconia, tungsten metal,

chromium metal, molybdenum metal, tungsten carbide, iron, tantalum carbide, zirconium boride, etc.

So far as has been found, no other additive of equal sodium content can replace the sodium hydroxide in the agglomerate material with equivalent results. The use of waterglass, i.e., sodium silicate, for example, promotes surface cracking or crazing of the molds and frequently results in bloating or puffing up of the molds during curing. This apparently results from the apparent blockage of the pores in the ceramic by the waterglass. As a result, the hydrogen gas formed during the reaction is unable to pass off without bloating the mold. Thus, the dimensional accuracy of the mold is destroyed and it becomes unusable for its primary purpose in the precision casting of metals.

The treatment of the silicon metal powder prior to use in the mold composition is of vital importance in carrying out this invention. The use of unfired silicon metal powder in the composition of this invention is impractical because the desired bonding reaction, as mentioned, takes place so rapidly as to preclude its commercial use as a molding material. With unfired silicon the bonding reaction starts to take place within a minute or so of mixing. The heat generated by the mixing operation speeds up this reaction. Furthermore, since the reaction is exothermic in nature, the larger the mass being mixed the more rapidly does the reaction take place. Therefore, in a large enough batch of materials to be considered commercially feasible, the reaction will go to completion before mixing is complete. Since this is a nonreversible chemical reaction, pressing or ramming of the mold must be completed before the bonding reaction commences. Otherwise, the act of pressing or ramming will destroy the mechanical properties of the bond as rapidly as the bond forms. When chemical bonding reaction takes place prior to or during the pressing or ramming operation, the mold has no usable strength. Hence, the practical use of this bonding system requires that the start of the chemical bonding reaction be inhibited without affecting its ultimate completion to a strong bond.

The key to the practical use of this invention is the discovery that it is possible to induce an incubation period of controlled length into the bonding reaction by prefiring the silicon metal under controlled conditions. It is necessary to conduct this prefiring operation in free air so that each particle of silicon powder becomes completely and uniformly coated with oxide. The time and temperature must be sufiicient for each particle to reach its equilibrium coating thickness of oxide. This oxide coating provides for a preliminary incubation period, during the total reaction period, which is a period during which the actual bonding reaction is delayed, apparently due to the necessity of dissolving the oxide film by chemical attack of the sodium hydroxide before the silicon metal core within the metal-metal-oxide particle can react with the sodium hydroxide in the final bonding reaction. This coating thickness, and therefore the incubation time of the reaction, is characteristic of the selected prefiring temperature rather than being a function of time and temperature. It is necessary, however, to hold the silicon powder at temperature long enough to insure a uniform temperature throughout the mass of powder and to insure adequate access of free air to the surface of each particle through either diifusion or mechanical motion. In practice, large quantities of silicon metal powder are fired for intervals of up to 16 hours to insure that equilibrium conditions for the given temperature have been reached throughout the mass. For example, after prefiring at 1800 F. for 16 hours, the silicon metal can be used in the composition of this invention and will have an incubation period of about 8 hours at room temperature, while still curing in about one-half hour at 400 F. This has been found to be a practical balance between having a sufliciently long incubation 4 period to permit economical mixing and molding procedures and having the molds take an excessive amount of time to cure.

The molds formed from this composition are hard, strong, dense, and readily machinable with carbide tools or by grinding after the curing operation and prior to the preheating operation. Since the molds do not change shape or size in any irreversible manner during preheating, except as such change is inherent in the refractory base powder chosen, the preheated mold and, hence, the casting will be representative of the exact shape machined into the mold in its cured state. Since the molds are hard and strong throughout, the cut or machined surfaces are normally of a very fine surface finish and result in castings having excellent surface finish. By means of this technique, very complex castings can be made without the necessity for forming expensive pattern plates and core dies. This technique also permits of greater accuracy in the forming of intricate castings than would be possible if a series of molds and cores produced independently were to be assembled into the finished mold. Ceramic mold compositions previously proposed have not been capable of being machined into such precision casting molds of great intricacy, accuracy, and high surface finish.

The sodium hydroxide content of the agglomerate material has been found to be somewhat critical. For forming steel castings, the practical limits are from, by weight, 1 to 40 parts of sodium hydroxide with 1000 parts of silica flour. If higher melting point refractory powders are used, still higher sodium hydroxide contents could be used although there is very little advantage to be gained by such increases. The reason for the upper limit of sodium hydroxide content is that excessive sodium hydroxide reduces the melting point of the material. It should be pointed out here that much higher sodium hydroxide contents are permissible in this composition than have been practical in previously proposed compositions. The reason for this is that in this composition there is much less tendency for the caustic to migrate to the surface of the mold. Hence, for any given percentage of caustic, the maximum local concentration is much lower in this invention than in previous compositions.

The bentonite can be varied from one percent to five percent by weight, that is, approximately 10 to 50 parts by weight of bentonite per 1000 parts by weight of silica flour. Below one percent the strength of the cured mold drops off substantially, and above five percent the mold starts to shrink appreciably and crack during curing and preheating. Less than one percent bentonite content also contributes to thermal cracking during mold preheat.

The content of prefired silicon metal powder can vary from one to ten percent by weight, that is, approximately 10 to parts by weight of prefired silicon metal powder per 1000 parts by weight of silica flour. Below one percent the mold loses so much strength that the advantages of this invention disappear. Above ten percent the amount of hydrogen formed during curing causes a serious bloating problem. A preferred range of prefired silicon metal powder content lies between two and four percent by weight based on silica flour equivalent weight.

Nonacidic bentonite is used in this invention. This bentonite is frequently known as Western bentonite. Acidic, or Southern, bentonite does not appear to be compatible with the sodium hydroxide contained in the agglomerate material. In particular, the substitution of acidic bentonite for nonacidic bentonite results in cured molds lacking in usable strength.

The water content of the agglomerate material is critical. Sufficient water must be employed to moisten thoroughly the total material or the cured mold will be substantially lacking in strength. On the other hand, ex-

cessive moisture content will cause the pressed mold to be spongy and lacking in dimensional stability. If still further moisture is added, the mix becomes a slurry and this bonding system has proved to be virtually useless in slurry form. The amount of moisture to be added must vary according to the fineness of the refractory powder, the percent of bentonite used, the chemical and surface characteristics of the refractory powder used, and the pressure to be used in forming the mold. If 200 mesh silica flour is used with two percent by weight of bentonite, with a molding pressure of 1000 pounds per square inch of mold surface, about fifteen percent moisture content by weight is optimum. The finer the particle size of the refractory powder used, and the higher the bentonite content used, the higher will be the moisture content required to obtain a usefully strong cured mold. Any tendency of the base refractory material to hydrate would also increase moisture requirements. The higher the molding pressure, the less moisture required. To meet the requirements discussed above, a water content of from 100 to 250 parts by weight per 1000 parts by weight of silica flour has been found suitable.

Powdered silicon metal as herein referred to includes the powders of pure silicon as well as certain other silicon alloys. For the purpose of this invention, the term powdered silicon metal shall mean and include the group consisting of silicon metal, high silicon ferro-silicon and calcium silicon alloys. With respect to high silicon ferro-silicon powders, it has been found desirable to employ alloys of at least seventy-five percent silicon and preferably more. Calcium silicon alloys have been found to be satisfactory where the silicon content is as low as fifty percent. In the powdered silicon metal as employed herein prefiring is believed to establish an outside coating which, in the mixture, must first be invaded before reaction can take place in curing the mold. Preferably the prefiring of the silicon metal powder should take place at temperatures between 400 F. and 2200 F.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.

What is claimed is:

1. A chemically bonded molded material for forming precision casting molds and the like consisting essentially of an agglomerate of approximately 1000 parts by weight of a refractory powder selected from the group consisting of silica flour and the silica flour equivalent weight of a refractory powder, from to 50 parts by weight of nonacidic bentonite, from 1 to 40 parts by weight of sodium hydroxide, from 10 to 100 parts of silicon metal powder, and from 100 to 250 parts of water where said silicon metal powder is prefired at a temperature of from 400 F. to 2200 F.

2. A chemically bonded molding material for forming precision casting molds and the like consisting essentially of an agglomerate of approximately 1000 parts by weight of silica flour, from approximately 10 parts to approximately 50 parts of nonacidic bentonite, from approximately 10 to 100 parts of fired silicon metal powder, from 1 to 40 parts of sodium hydroxide, and from approximately 100 to 250 parts by weight of water.

3. Molding material for forming pressed ceramic molds and the like consisting essentially of an agglomerate of approximately 1000 parts by weight of approximately 200 mesh silica flour, approximately 40 parts by weight of nonacidic bentonite, approximately 4 parts by weight of sodium hydroxide, approximately 20 parts by weight of silicon metal powder prefired in air to 1800 F. for 16 hours, and approximately 180 parts by weight of water; the bentonite, sodium hydroxide, and prefired silicon metal having a chemical reaction productive of a high strength ceramic bond at room temperature, which chemical reaction is susceptible of being accelerated by the application of heat through the heating of the mold, and

which reaction is by nature exothermic so that once it starts it is self-propagating through the thickness of the mold.

4. A chemically bonded molding material for forming precision casting molds and the like consisting of a thoroughly mixed aggregate of approximately 1000 parts by weight of a refractory powder selected from the group consisting of silica flour and the silica flour equivalent weight of a refractory powder, from 10 parts to 50 parts by weight of nonacidic bentonite, from 10 parts to parts of fired silicon metal powder, from 1 to 40 parts by weight of sodium hydroxide, and from 100 to 250 parts by Weight of water.

5. A method of predetermining the incubation portion of the total time required for the reaction between a mixture of silicon metal powder, silica flour, bentonite, sodium hydroxide, and water, which comprises prefiring said silicon metal powder in free air to a temperature of from 400' F. to 2200 F. and distributing the prefired silicon metal powder throughout said mixture whereby the incubation time is proportional to the temperature at which said silicon metal powder is prefired.

6. A method of fixing the incubation portion of the total time required for the reaction between a mixture of silicon metal powder, bentonite, sodium hydroxide, and water to approximately 8 hours, and for completion of curing at room temperature within 24 hours and within /z-hour at 400 F., which comprises prefiring said silicon metal powder to 1800 F. and distributing the prefired silicon metal powder throughout the mixture.

7. A method of predetermining the incubation portion of the total time required for the reaction between silicon metal powder, sodium hydroxide, and water which comprises prefiring of the silicon metal powder in free air at a temperature determined by the incubation period desired in the reaction prior to the admixture of the water and sodium hydroxide where the prefiring temperature is between 400 F. and 2200 F. and subsequently mixing it with the water and the sodium hydroxide.

8. A method offixing the incubation portion of the total time required for the reaction between a composition of nonacidic bentonite, sodium hydroxide, water, and prefired silicon metal powder being used to chemically bond a refractory powder at approximately 8 hours for the initiation of curing at room temperature and completion of curing at room temperature at approximately 24 hours, and with the corresponding time for completion of curing at 400 F. at approximately /2-iour, which comprises prefiring the silicon metal to 1800 F. for 16 hours in free air prior to the admixture of the silicon metal powder to the other reactant ingredients, and subsequently mixing it with said ingredients.

9. The method of producing a ceramic article particularly for use as a precision casting mold which consists of the steps of thoroughly mixing a dry composite consisting essentially of approximately 1000 parts by weight of a refractory selected from the group consisting of silica flour and the silica flour equivalent weight of a refractory powder, with 40 parts by weight of nonacidic bentonite, and 20 parts by weight of silicon metal powder prefired to 1800 F. for 16 hours in air; dissolving 4 parts by weight of sodium hydroxide in 180 parts of water, agglomerating the wet and dry ingredients, firmly pressing the damp agglomerate into a molding flask about a pattern, removing the pattern and flask from the molded article, and curing the mold, the bentonite, sodium hydroxide, and prefired silicon metal chemically reacting to form a strong, waterproof bond throughout the article.

10. The method of producing a ceramic article particularly for use as a precision casting mold which consists of the steps of thoroughly mixing a dry composite consisting essentially of approximately 1000 parts by weight of a refractory selected from the group consising of silica flour and the silica flour equivalent weight of a refractory powder, with 40 parts by weight of nonacidic bentonite, and 20 parts by weight of silicon metal powder prefired to 1800 F. for 16 hours in air; dissolving 4 parts by weight of sodium hydroxide in 180 parts of water, agglomerating the wet and dry ingredients, firmly pressing the damp agglomerate into a molding flask about a pattern, removing the pattern and flask from the molded aricle, and curing the mold at room temperature, the bentonite, sodium hydroxide, and prefired silicon metal chemically reacting to form a strong, waterproof bond throughout the article.

11. The method of producing a ceramic article particularly for use as a precision casting mold capable of being preheated to temperatures in the range of about 2400 P. which consists of the steps of thoroughly mixing a dry composite consisting essentially of approximately 1000 parts by weight of a refractory selected from the group consisting of silica flour and the silica flour equivalent weight of a refractory powder, with 40 parts by weight of nonacidic bentonite, and 20 parts by weight of silicon metal powder prefired to 1800 F. for 16 hours in air; dissolving 4 parts by weight of sodium hydroxide in 180 parts of water, agglomerating the wet and dry ingredients, firmly pressing the damp agglomerate into a molding flask about a pattern, removing the pattern and flask from the molded article, and curing the mold at a temperature up to 800 F. to accelerate the curing reaction, the bentonite, sodium hydroxide, and prefired silicon metal chemically reacting to form a strong, waterproof bond throughout the article.

References Cited in the file of this patent UNITED STATES PATENTS 1,959,179 Snell May 15, 1934 2,241,354 Lowe May 6, 1941 2,586,814 Greenewald Feb. 26, 1942 2,997,400 Greenewald Aug. 22, 1961 3,017,677 Greenewald Jan. 23, 1962 

1. A CHEMICALLY BONDED MOLDED MATERIAL FOR FORMING PRECISION CASTING MOLDS AND THE LIKE CONSISTING ESSENTIALLY OF AN AGGLOMERATE OF APPROXIMATELY 1000 PARTS BY WEIGHT OF A REFRACTORY POWDER SELECTED FROM THE GROUP CONSISTING OF SILICA FLOUR AND THE SILICA FLOUR EQUIVALENT WEIGHT OF A REFRACTORY POWDER, FROM 10 TO 50 PARTS BY WEIGHT OF NONACIDIC BENTONITE, FROM 1 TO 40 PARTS BY WEIGHT OF SODIUM HYDROXIDE, FROM 10 TO 100 PARTS OF SILICON METAL POWDER, AND FROM 100 TO 250 PARTS OF WATER WHERE SAID SILICON METAL POWDER IS PREFIRED AT A TEMPERATURE OF FROM 400*F. TO 2200*F. 